Add keyboard 'UM-80' (#17035)

keyboards/merge/um80/readme.md

@@ -0,0 +1,22 @@
+# UM-80
+
+![UM-80](https://i.imgur.com/JVEC4Ntl.jpeg)
+
+UM-80 is a split keyboard in 75% layout with RGB, macro keys, OLED screen and encoder housed in PLA 3D printed cases sold by Merge. [Product page](https://mergedesign.store/products/um-80)
+
+* Keyboard Maintainer: [duoshock](https://github.com/duoshock)
+* Hardware Availability: [Merge Store](https://mergedesign.store/products/um-80)
+
+Make example for this keyboard (after setting up your build environment):
+
+    make merge/um80:default
+
+See the [build environment setup](https://docs.qmk.fm/#/getting_started_build_tools) and the [make instructions](https://docs.qmk.fm/#/getting_started_make_guide) for more information. Brand new to QMK? Start with our [Complete Newbs Guide](https://docs.qmk.fm/#/newbs).
+
+## Bootloader
+
+Enter the bootloader in 3 ways:
+
+* **Bootmagic reset**: Hold down the Esc key on the left side or '7' key on the right side and plug in the keyboard.
+* **Physical reset button**: Briefly press the button on the back of the PCB.
+* **Keycode in layout**: Press the key mapped to `RESET` if it is available.